1. Field of the Invention
The invention relates to a drive apparatus useful in screw injection molding units as well as other apparatus using simultaneous rotation around and displacement along an axis.
2. Description of Related Technology
A drive apparatus is disclosed in U.S. Pat. No. 4,005,641 (equivalent to DE No. 24 61 854) whose disclosure is incorporated herein by reference. The drive apparatus described has an extruder screw rigidly connected to a drive piston. This piston may rotate as well as displace along the rotation axis. Inside the piston is a drive shaft which engages the drive piston by a plurality of splines or ridges so that the shafts may rotate together but only the drive piston has axial movement. The drive shaft is connected to a motor shaft.
Screw injection molding processes are a particularly useful application for drive apparatus such as that of U.S. Pat. No. 4,005,641 because these processes have two stages where pressure from the drive apparatus is required. The first phase is in metering plastic material into the injection head. Typically, relatively low pressures are required and hydraulic pressure would be applied, in the U.S. Pat. No. 4,005,641 drive, to the smaller diameter of the drive shaft through a bore in the drive shaft.
The second phase is the injection stroke. The extruder screw is extended by applying pressure across the areas of both the drive shaft and piston to force plasticized material into a mold. By rotating both the shaft and piston, the screw may be rotated during the injection stroke.
In U.S. Pat. No. 4,005,641, the use of a bore in the drive shaft requires the use of shaft diameters sufficiently large to provide material strength and sufficient hydraulic area. As the drive piston diameter increases, the diameters of the sealing gaskets increases. Gaskets with larger diameters are subject to higher linear speeds and forces than those of smaller diameters. These stresses lead to failure and required maintenance.